Question

我正在为我的课程编写课程。我是C ++的新手。我们尝试做的是在执行时提供用户提供的年，月和日参数，并计算它们与当前日期之间的差异。

在我的main函数中，我有以下代码从用户那里获取这些参数：

int main(int argc, char** argv)

我的理解是，这会创建一个字符数组并将用户的参数存储在其中。接下来，我将存储在此数组中的参数分配给新的char变量，以便我可以将它们传递给函数，如下所示：

int main(int argc, char** argv)
{
    int year, month, day;
    char arg1 = argv[1];
    string arg2 = argv[2];
    char arg3 = argv[3];

    argumentChange(argc, arg1, arg2, arg3, year, month, day);

blablabla other code
}

argumentChange是我发送给他们的功能。问题是当我尝试编译时，我收到以下错误：

错误：来自＆＃39; char *＆＃39;的转换无效去＃char;＆＃39; [-fpermissive]

charArg1 = argv [1];

错误：来自＆＃39; char *＆＃39;的转换无效去＃char;＆＃39; [-fpermissive]

charArg3 = argv [3];

我已经尝试过搜索这个问题，但我无法对其他地方的解释做出正面或反面的讨论。我已经看到很多人提到＆＃34;指针＆＃34;，我们还没有在课堂上报道，而我对此一无所知。我做错了什么？我如何使这项工作？

以下是我整个计划的完整代码：

#include <iostream>
#include <ctime>
#include <string>

using namespace std;

void argumentChange(int numArg, char chArg1, string sArg2, char chArg3, int year, int month, int day)
{
    cout << "Starting birth date: " << chArg1 << " " << sArg2 << " " << chArg3
         << endl;
    if (numArg < 4 || numArg > 4)
    {
        cout << "Correct Usage: ./cl <birth_year><birth_month><birth_day>" << endl;
        cout << "Try again, dude." << endl;
    }
    else if (numArg == 4)
    {
        year = chArg1 - '0';
        day = chArg3 - '0';
        if ((sArg2 == "january") || (sArg2 == "January"))
        {
        month = 0;
        }
        else if ((sArg2 == "february") || (sArg2 == "February"))
        {
            month = 1;
        }
        else if ((sArg2 == "march") || (sArg2 == "March"))
        {
            month = 2;
        }
        else if ((sArg2 == "april") || (sArg2 == "April"))
        {
            month = 3;
        }
        else if ((sArg2 == "may") || (sArg2 == "May"))
        {
            month = 4;
        }
        else if ((sArg2 == "june") || (sArg2 == "June"))
        {
            month = 5;
        }
        else if ((sArg2 == "july") || (sArg2 == "July"))
        {
            month = 6;
        }
        else if ((sArg2 == "august") || (sArg2 == "August"))
        {
            month = 7;
        }
        else if ((sArg2 == "september") || (sArg2 == "September"))
        {
            month = 8;
        }
        else if ((sArg2 == "october") || (sArg2 == "October"))
        {
            month = 9;
        }
        else if ((sArg2 == "november") || (sArg2 == "November"))
        {
            month = 10;
        }
        else if ((sArg2 == "december") || (sArg2 == "December"))
        {
            month = 11;
        }
        else
        {
                cout << "Error: You have entered an invalid term for month.  Please type ";
                cout << "the complete name of a valid month." << endl;
        }
    }
}

struct tm bday(int year, int month, int day)
    {
        struct tm r {0};

        r.tm_year = year - 1900;
        r.tm_mon = month;
        r.tm_mday = day;

        return r;
    }

int main(int argc, char** argv)
{
    int year, month, day;
    char arg1 = argv[1];
    string arg2 = argv[2];
    char arg3 = argv[3];

    argumentChange(argc, arg1, arg2, arg3, year, month, day);

    struct tm a = bday(year, month, day);
    time_t x = mktime(&a);
    time_t y = time(0);
    if ( x != (time_t)(-1) && y != (time_t)(-1) )
    {
        double difference = difftime(y, x) / (60 * 60 * 24);
        cout << ctime(&x);
        cout << ctime(&y);
        cout << "difference = " << difference << " days" << endl;
    }
    return 0;
}

Answer 1

正如所指出的，argv [x]的类型是char*，它是指向字符数组的指针。所以char**是指向数组数组的指针。（真的*并不自动意味着它是指向数组的指针，在这种情况下就是这样）

//  argv is an array of char*'s
int main(int argc, char** argv)

// firstParam is an array of char's 
char* firstParam = argv[1];

// firstLetter is the first char of the array
char firstLetter = firstParam[0]

实际上，您应该跳过所有char数组，然后使用std::string＆＃39}。例如

int main(int argc, char** argv) {
    std::string param = argv[1];

    if (param == "january") {
        // Something ...
    }

}

您还应该检查传入的参数数量，因此您无法访问数组范围之外的内容。这是argc的用途。

Answer 2

在T类型中存储一堆变量时，我们可能会有dynamic array来存储它。通常我们使用指向数组的指针，其类型为T*

以int为例：

int* ary1 = new int[3];
ary1[0] = 10; // ary1 + 0 store 10, a int
ary1[1] = 11; // ary1 + 1 store 11, a int 
ary1[2] = 12; // ary1 + 2 store i2, a int

与char*字符串

相同

char** argv= new char*[4];
argv[0] = "cl.exe" // ary2 + 0 stores "cl.exe", a `char*` string
argv[1] = "2018""  // ary2 + 1 stores "2018"  , a `char*` string
argv[2] = "3"      // ary2 + 2 stores "3"     , a `char*` string
argv[3] = "16"     // ary2 + 3 stores "16"    , a `char*` string

因此，您无法将argv[1]，（char*字符串）分配给arg1，（char字符）。

C ++：＆＃39; char *＆＃39; vs＆＃39; char＆＃39;，在尝试分配变量时出错

2 个答案: